Bias circuit

ABSTRACT

A bias circuit for generating bias voltages or bias currents including first and second elements for generating a voltage corresponding to the sum of two voltage drops in a forward p-n junction; first and second transistors for generating a negative feedback current; at least one resistor for determining the value of a constant current for generating bias voltages; a negative feedback circuit; a third resistor connected in the feedback circuit, and; a starting element for supplying currents to the first and second elements and to the first and second transistors in an initial state when the power is turned on, whereby the feedback circuit operates to generate the constant current which is used for forming bias voltages.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates generally to a bias circuit and, moreparticularly, to a bias circuit for generating bias voltages, i.e.,constant voltages, which may be, for example, supplied to the inputs ofa pre---amplifier and to the inputs of an input stage of a poweramplifier, in an audio-amplifier.

(2) The Prior Art

In a pre-amplifier or an input stage of a power amplifier, in order tostabilize its operation, it is necessary to apply stable bias voltagesto the inputs of the pre-amplifier or the like. In addition, in recentyears, the pre-amplifier or the like has been manufactured by usingsemiconductor technology, especially, integrated circuit technology, sothe bias voltages applied to the pre-amplifier or the like arerelatively low. Therefore, it is necessary to apply low and stable biasvoltages to the pre-amplifier or the like.

One prior art bias circuit for generating bias voltages which may be,for example, applied to the inputs of a pre-amplifier or the like, iscomposed of diodes whose forward voltages serve as bias voltages.However, the change of the voltage applied to the diodes varies theforward resistance of the diodes, i.e., varies the forward voltages ofthe diodes. Therefore, the bias voltages generated from the bias circuitformed by diodes are unstable with respect to the change of the powervoltage supplied to the bias circuit.

Another prior art bias circuit is composed of Zener diodes whose Zenervoltages serve as bias voltages. The bias voltages generated from thebias circuit formed by Zener diodes are stable with respect to thechange of the power voltage supplied to the bias circuit. However, it isdifficult to generate a low bias voltage, for example, less than 5volts.

SUMMARY OF THE INVENTION

Therefore, a principal object of the present invention is to provide abias circuit for generating low bias voltages which are stable withrespect to the change of the power voltage supplied to the bias circuit.

It is another object of the present invention to provide a bias circuitfor generating bias currents which are stable with respect to the changein power voltage supplied to the bias circuit.

The present invention provides a bias circuit for generating biasvoltages or bias currents comprising: first and second elements forgenerating a voltage corresponding to the sum of two voltage drops in aforward p-n junction; first and second transistors for setting anegative feedback current; at least one resistor for determining thevalue of a constant current for generating bias voltages; a negativefeedback circuit; a third resistor connected in the feedback circuit,and; a starting element for supplying currents to the first and secondelements and to the first and second transistors in an initial statewhen the power is turned on, whereby the feedback circuit operates togenerate the constant current which is used for forming bias voltages.

The present invention will be more apparent from the followingdescription with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a first embodiment of the biascircuit according to the present invention;

FIG. 2 is a graph showing the relationship between the current I₃ andthe currents I₄ and I₆, appearing in FIG. 1:

FIG. 3 is a circuit diagram illustrating the bias circuit of FIG. 1connected to a pre-amplifier of an audio-amplifier;

FIG. 4 is a circuit diagram illustrating a second embodiment of the biascircuit according to the present invention;

FIG. 5 is curves showing the relations between the current I₃ and thecurrents I₄ and I₆, appearing in FIG. 4;

FIG. 6 is a circuit diagram illustrating the bias circuit of FIG. 4connected to a pre-amplifier of an audio-amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the bias circuit comprises a starting resistor R₁,a first circuit formed by a transistor Q₃, a resistor R₃ and atransistor Q₅ connected in series, a second circuit formed by twotransistors Q₄ and Q₆ and a resistor R₄ connected in series, and a thirdcircuit for supplying constant currents to the first and secondcircuits, formed by two transistor Q₁ and Q₂. V_(cc), V_(R) and GNDindicate, a d.c. supply terminal, an output terminal of the bias circuitand a grounded terminal, respectively. C₁ is a phase-compensatingcapacitor for preventing the generation of a positive feedback by thephase shift of a high frequency which may invite the oscillation of thenegative feedback loop composed of the transistors Q₁, Q₆ Q₄ and Q₂ anda resistor R₂. Therefore, if there is no possibility of suchoscillation, the phase-compensating capacitor C₁ can be omitted. R₂ is aresistor for restricting the base current of the transistor Q₁ andstabilizing the gain of the negative feedback loop.

The operation of the bias circuit of FIG. 1 is as follows.

When the d.c. supply is turned on, a forward voltage is applied to theemitter of the transistor Q₁. However, since the transistor Q₂ does notconduct yet, the base current of the transistor Q₁ does not flow, sothat the transistor Q₁ does not conduct. Therefore, current suppliedfrom the d.c. supply flows only through two paths, one of which iscomposed of the resistor R₁, the transistor Q₃, the resistor R₃ and thetransistor Q₅, while the other is composed of the resistor R₁, thetransistor Q₄, the transistor Q₆ and the resistor R₄. These transistorsQ₃, Q₄, Q₅ and Q₆ are selected so as to satisfy the formulas (1) and (2)below.

    S.sub.E4 >S.sub.E3                                         (1)

    S.sub.E5 =S.sub.E6                                         (2)

where S_(E3), S_(E4), S_(E5) and S_(E6) are the emitter areas of thetransistors Q₃, Q₄, Q₅ and Q₆, respectively. As a result, as shown inFIG. 2, in an initial state wherein the collector current I₃ of thetransistor Q₃ is relatively low, the collector current I₄ of thetransistor Q₄ is greater than the collector current I₆ of the transistorQ₆. Therefore, a current I_(B) which equals (I₄ -I₆) is supplied to thebase of the transistor Q₂, so that the transistor Q₂ conducts. Thisconduction of the transistor Q₂ causes the base current of thetransistor Q₁ to flow. As a result, the transistor Q₁ also conducts.Therefore, the current which is supplied from the d.c. supply terminalV_(cc) to the transistors Q₃ and Q₄ flows through the transistor Q₁, notthrough the starting resistor R₁. Thus, the operation of the startingresistor R₁ is completed. In order to maintain the transistor Q₁,conductive it is preferable that the value of the starting resistor R₁be selected between the value of the saturation resistance and the valueof the cut-off resistance in the transistor Q₁. It should be noted thata junction FET (Field Effect Transistor), or other switching means whichmay effect the above-mentioned operation, can also be employed insteadof the resistor R₁.

In the bias circuit of FIG. 1 in which the transistor Q₁ is conducting,the base current I_(B) of the transistor Q₂ is selected to be small andneglible compared with the currents I₄ and I₆. The actual value of thecurrent I_(B) is I₁ /ββ' where I₁ is the collector current, and β and β'are the current amplification factors of the transistors Q₁ and Q₂,respectively. As a result, the current I₄ and the current I₆ are nearlyequal. This corresponds to the state when the current I₃ equals I₃ (OP)in FIG. 2. In addition, the current I₆ is fixed to be a constant valueby the collector-base connected transistor Q₅, which serves as a diode.This causes the currents I₁ and I₂ to be constant, so that the currentsI₃ and I₅, which are equal, are also constant. Therefore, a voltage atthe point A, namely, at the terminal V_(R) , is constant, which voltageV_(R1) can be expressed as follows.

    V.sub.R1 =V.sub.BE3 +V.sub.BE5                             (3)

where V_(BE3) and V_(BE5) are the base-emitter voltages of thetransistors Q₃ and Q₅, respectively. The bias voltage V_(R1) is aconstant voltage regardless of the change of the d.c. supply voltage,since such a change of voltage is absorbed in the emitter-collectorvoltage V_(EC) of the transistor Q₁.

The selection of the transistors Q₃ through Q₆ and the resistors R₃ andR₄ of FIG. 1 will be explained below.

V_(BE3) and V_(BE5) in the equation (3) can be expressed as follows.

    V.sub.BE3 =I.sub.3 R.sub.3 +V.sub.BE4                      (4)

    V.sub.BE5 =I.sub.6 R.sub.4 +V.sub.BE6                      (5)

where V_(BEi) is the base-emitter voltage of the transistor Qi(i equals3 through 6). In general, ##EQU1## where k: Boltzmann's constant

T: Absolute temperature

q: Charge

I_(c) : Collector current

I_(s) : Saturated base-emitter current.

Then, the equations (4) and (5) can be rewritten as follows. ##EQU2##where I_(si) is the saturated base-emitter current of the transistor Qi(i equals 3 through 6). In general, in a monolithic integrated circuitmanufactured on a chip, the saturated base-emitter current of atransistor is in proportion to the area of the emitter of thetransistor. When a plurality of transistors whose emitter areas are thesame are connected in parallel, the saturated base-emitter current as awhole is in proportion to the number of the parallel-connectedtransistors. Therefore, defining two constants n₁ and n₂ as I_(s4)/I_(s3) and I_(s5) /I_(s6), respectively, and using the condition thatI₃ =I₅ and I₄ =I₆, the equation (9) is obtained. ##EQU3## Thus, asunderstood from the equation (9), the constant value of the current I₃can be determined by selecting four values, namely, the two constants n₁and n₂ depending upon the emitter areas of the transistors Q₃ through Q₆and the two values of the resistor R₃ and R₄. As a result, the voltageV_(R1) at the point A becomes a constant voltage.

In the bias circuit of FIG. 1, since a constant voltage can be obtainedby causing the current I₃ to be constant, a plurality of constantvoltages can also be obtained by inserting one or more diodes betweenthe points B and C. For example, a low bias voltage which is n-times ofa base-emitter voltage V_(BE) of a transistor can be obtained byinserting a plurality of transistors which serve as diodes.

In FIG. 1, a transistor Q₇, whose base and emitter are connected to thebase of the transistor Q₁, and the d.c. supply terminal V_(cc),respectively, serves as a constant d.c. supply source. The collectorcurrent of the transistor Q₇ is a constant current which is inproportion to the current I₁ and the ratio of the emitter area of thetransistor Q₇ to that of the transistor Q₁.

FIG. 3 is a circuit diagram illustrating the bias circuit of FIG. 1connected to a pre-amplifier of an audio-amplifier. In FIG. 3, theelements illustrated in FIG. 3 which are identical with those of FIG. 1are given the same reference numerals as in FIG. 1. In FIG. 3, the biascircuit B₁ further includes two transistors Q₈ and Q₉ which serve asdiodes, in addition to the bias circuit of FIG. 1, so that four biasvoltages V_(R1), V_(R2), V_(R3) and V_(R4) are obtained at the terminalsA, B, B' and C, respectively. The pre-amplifier of FIG. 3 is composed ofa signal source SS, a capacitor C₁₁ for cutting off the direct-currentcomponent of the input signal, an input resistor R₁₁, an inputtransistor Q₁₂, two bias transistors Q₁₁ and Q₁₃ for the inputtransistor Q₁₂, two Darlington-connected transistors Q₁₄ and Q₁₅ whichserve as an amplifier, a bias transistor Q₇ for the two transistors Q₁₄and Q₁₅, and two output transistors Q₁₆ and Q₁₇ connected in series. Inthis case, the bias transistor Q₇ supplies a bias current, which isstable regardless of the change of the d.c. supply voltage, to thetransistors Q₁₄ and Q₁₅. The terminal V.sub. out between the outputtransistors Q₁₆ and Q₁₇ is an output terminal of the pre-amplifier.Thus, the bias circuit of FIG. 1 is applicable to a pre-amplifier whichneeds a plurality of bias voltages and a bias current.

FIG. 4 is a circuit diagram illustrating a second embodiment of the biascircuit according to the present invention. In FIG. 4, the elementsillustrated in FIG. 4 which are identical with those of FIG. 1 are giventhe same reference numerals as in FIG. 1. Although the bias circuit ofFIG. 1 includes two resistors R₃ and R₄ for determining the magnitude ofa constant current therein, the bias current of FIG. 4 includes aresistor R₅ instead of the resistor R₃ and R₄.

The operation of the bias circuit of FIG. 4 is as follows.

In the same way as mentioned in the case of FIG. 1, when the d.c. supplyis turned on, current supplied by the d.c. supply flows only through twopaths, one of which is composed of the resistor R₁, the transistors Q₃and Q₅ and the resistor R₅, while the other is composed of the resistorR₁, and the transistors Q₄ and Q₆. These transistors Q₃, Q₄, Q₅ and Q₆are selected so as to satisfy the formulae (1)' and (2)' as follows.

    S.sub.E4 >S.sub.E3                                         (1) '

S_(E5) >S_(E6) (2) '

where S_(E3), S_(E4), S_(E5) and S_(E6) are the emitter areas of thetransistors Q₃, Q₄, Q₅ and Q₆, respectively. As a result, as shown inFIG. 5, in an initial state, wherein the collector current I₃ of thetransistor Q₃ is relatively low, the collector current I₄ of thetransistor Q₄ is greater than the collector current I₆ of the transistorQ₆. Therefore, a current I_(B) which equals (I₄ -I₆) is supplied to thebase of the transistor Q₂, so that the transistor Q₂ conducts, and afterthat, the transistor Q₁ conducts. Then, the current which is suppliedfrom the d.c. supply terminal V_(cc) to the transistors Q₃ and Q₄ flowsthrough the transistor Q₁, not through the resistor R₁. Thus, theoperation of the starting resistor R₁ is completed.

In the bias circuit of FIG. 4, in which the transistor Q₁ is conducting,the base current I_(B) of the transistor Q₂ is selected to be negligiblecompared with the currents I₄ and I₆. As a result, the current I₄ andthe current I₆ are nearly equal, which corresponds to the state when thecurrent I₃ equals I₃ (OP) in FIG. 5. In addition, the current I₆ isfixed to be a constant value by the collector-base connected transistorQ₅, which serves as a diode. This causes the currents I₁ and I₂ to beconstant, so that the currents I₃ and I₅ are also constant. Therefore, avoltage at the point A, namely, at the terminal V_(R), is constant, andthe voltage V_(R1) can be expressed as follows.

    V.sub.R1 =V.sub.BE4 +V.sub.BE6                             (3)'

where V_(BE4) and V_(BE6) are the base-emitter voltages of thetransistors Q₄ and Q₆, respectively.

The selection of the transistors Q₃ through Q₆ and the resistor R₅ ofFIG. 4 will be explained below.

V_(BE4) and V_(BE6) in the equation (3)' can be expressed as follows.

    V.sub.BE4 =V.sub.BE3                                       (4)'

    V.sub.BE6 =V.sub.BE5 +I.sub.5 R.sub.5                      (5)'

where V_(BEi) is the base-emitter voltage of the transistor Qi (i equals3 through 6). In general, as mentioned above, in the equation (6)##EQU4##

Then, the equations (4)' and (5)' can be rewritten as follows. ##STR1##Therefore, defining two constant n₁ and n₂ as I_(s4) /I_(s3) and I_(s5)I_(s6), respectively, and using the condition that I₃ =I₅ and I₄ =I₆,the equation (9)' is obtained. ##EQU5## Thus as understood from theequation (9)', the constant value of the current I₃ which equals thecurrent I₅ can be determined by selecting three values namely, the twoconstant n₁ and n₂, depending upon the emitter areas of the transistorsQ₃ through Q₆ and the value of the resistor R₅. As a result, the voltageV_(R1) at the point A becomes a constant voltage.

In the bias circuit of FIG. 4, a plurality of constant voltages can alsobe obtained by inserting one or more diodes between the points B and C.

FIG. 6 is a circuit diagram illustrating the bias circuit of FIG. 4connected to a pre-amplifier of an audio-amplifier. In FIG. 6, theelements illustrated in FIG. 6 which are identical with those of FIG. 4given the same reference numerals as in FIG. 4. In FIG. 6, the biascircuit B₂ further includes a transistor Q₁₀, in addition to the biascircuit of FIG. 4, so that three bias voltages V'_(R1), V'_(R2) andV'_(R3) are obtained at the terminals A, B and C, respectively. Thepre-amplifier of FIG. 6 is composed of a signal source SS, a capacitorC₂₁ for cutting off the direct-current component of the input signal, aninput resistor R₂₁, an input transistor Q₂₂, two bias transistors Q₂₁and Q₂₃ for the input transistor Q₂₂, two Darlington-connectedtransistors Q₂₄ and Q₂₅, which serve as an amplifier, a bias transistorQ₇ for two transistors Q₂₄ and Q₂₅, and two output transistors Q₂₆ andQ₂₇ connected in series. Also, in this case, the bias transistor Q₇supplies a bias current, which is stable regardless of the change of thed.c. supply voltage, to the transistors Q₂₄ and Q₂₅. The terminalV_(out) between the output transistor Q₂₆ and Q₂₇ is an output terminalof the pre-amplifier. R₂₂ and R₂₃ are resistors for determining theratio of the feedback of the pre-amplifier and C₂₂ is a capacitor forcutting off the direct-current component in the feedback current of thepre-amplifier. C₂₃ is a capacitor for cutting off the high-frequencycomponent of the input signal. Thus, the bias circuit of FIG. 4 isapplicable to a pre-amplifier which needs a plurality of bias voltagesand a bias current.

As explained hereinbefore, the bias circuit according to the presentinvention has the following advantages as compared with those of theprior art.

(1) A bias voltage generated from the bias circuit is stable regardlessof the change of the d.c. supply V_(cc), since such a change can beabsorbed by the transistor Q₁.

(2) A bias voltage generated from the bias circuit is relatively low,since the bias circuit can generate a constant current which causes thebias voltage by using diodes or the like whose forward resistance isrelatively small.

(3) The bias circuit also can generate a bias current which is stableregardless of the change of the d.c. supply V_(cc).

What is claimed is:
 1. A bias circuit, having a d.c. supply terminal, abias output terminal, and a reference potential terminal,comprising:first and second elements arranged in series between the biasoutput terminal and the reference potential terminal of said biascircuit, said first and second elements generating a voltage between thebias output terminal and the reference potential terminal, said voltagebeing equal to the sum of two voltage drops in a forward p-n junction;first and second transistors connected in series, the collectors of saidfirst and second transistors being connected at a node, each of saidfirst and second transistors having a base which is connected to aterminal between said first and second elements, wherein said first andsecond transistors generate a difference current at said node equal tothe difference between the collector currents of said first and secondtransistors; resistor means, operatively connected to said secondelement, for determining the value of a constant current for generatingbias voltages; a third transistor arranged between the bias outputterminal and the d.c. supply terminal; a negative feedback circuitarranged between the base of said third transistor and said node, saiddifference current being introduced as an input thereto; a startingelement, connected to the d.c. supply terminal, for supplying currentsto said first and second elements and to said first and secondtransistors in an initial state when the power is turned on, wherebysaid negative feedback circuit operates to generate said constantcurrent.
 2. A bias circuit as set forth in claim 1, wherein saidresistor means comprises a first resistor connected between said firstand second elements and a secnod resistor operatively connected to saidsecond transistor.
 3. A bias circuit as set forth in claim 1, whereinsaid resistor means comprises of a single resistor connected to saidsecond element.
 4. A bias circuit as set forth in claim 1, wherein saidbias circuit further comprises a fourth transistor whose base andemitter are connected to the base and the emitter of said thirdtransistor, respectively, whereby a collector current of said fourthtransistor may be obtained as a bias current.
 5. A bias circuit as setforth in claim 1, further comprising at least one diode arranged betweensaid first and second elements, whereby a plurality of bias voltages maybe output from the terminals of said at least one diode.
 6. A biascircuit as set forth in claim 5, wherein said at least one diode is abase-collector connected transistor.
 7. A bias circuit as set forth inclaim 1, wherein each of said first and second elements is a transistor.8. A bias circuit as set forth in claim 1, wherein said starting elementis a resistor.
 9. A bias circuit, having a voltage supply terminal, aground terminal, and an output terminal, comprising:a first transistor,having a collector, emitter and base, said first transistor emitteroperatively connected to the voltage supply terminal and said firsttransistor collector operatively connected to the output terminal; afirst resistor operatively connected between the voltage supply terminaland the output terminal; a second transistor, having a collector,emitter and base, said second transistor collector operatively connectedto said first transistor base; a second resistor operatively connectedto said second transistor emitter and to the ground terminal; a firstcapacitor operatively connected to said second transistor base and tothe ground terminal; a third transistor, having a collector, emitter andbase, said third transistor emitter operatively connected to the outputterminal and said third transistor base operatively connected to saidthird transistor collector; a fourth transistor, having a collector,emitter and base, said fourth transistor emitter operatively connectedto the output terminal, said fourth transistor base operativelyconnected to said third transistor collector and said third transistorbase, and said fourth transistor collector operatively connected to saidsecond transistor base and said first capacitor; a fifth transistor,having a collector, emitter and base, said fifth transistor base andcollector operatively connected to said third transistor base, saidfifth transistor emitter operatively connected to the ground terminal; asixth transistor, having a collector, emitter and base, said sixthtransistor collector operatively connected to said fourth transistorcollector, said second transistor base, and said first capacitor, saidsixth transistor base operatively connected to said fifth transistorbase and collector, said sixth transistor emitter operatively connectedto the ground terminal; and a seventh transistor, having a collector,emitter and base, said seventh transistor emitter operatively connectedto the voltage supply terminal, said seventh transistor base operativelyconnected to said first transistor base and said second transistorcollector.
 10. The bias circuit as set forth in claim 9, furthercomprising:a third resistor operatively connected between said thirdtransistor collector and base; and a fourth resistor operativelyconnected between said sixth transistor emitter and the ground terminal.11. The bias circuit as set forth in claim 10, further comprising atleast one diode operatively connected between said third resistor andsaid fifth transistor collector.
 12. The bias circuit as set forth inclaim 9, further comprising a third resistor operatively connectedbetween said fifth transistor emitter and the ground terminal.